To use Billy Commercially you must first purchase a commercial license. You can pay any amount you please - consider it a 'pay what you want' situation! Buy Billy by clicking here. Once you've purchased it, you are free to use Billy in your commercial projects.
[UPDATE 2022] Btw, Billy 2.0 is now available! Improved kerning, spacing, 350 glpyhs (full Latin) and more: download here.